1. Field of the Invention
The present invention relates to a method for improving the magnetic properties of metallic glass ribbon, particularly by removing a portion of the ribbon surface prior to magnetic annealing.
2. Description of the Prior Art
For purpose of the present invention, a ribbon is a slender body of substantially rectangular cross section whose transverse dimensions are much smaller than its length.
Continuous ribbon of metallic glass can be formed by impinging a molten stream of certain metal alloys onto a rapidly moving chill block; for example, a rapidly rotating, water-cooled wheel. The molten metal is quenched rapidly into a glassy state on the chill surface. Before it solidifies, however, air may be trapped between the molten metal and the chill block surface. This trapped air results in some regions of the ribbon surface that adjoins the chill surface being quenched while out of contact with the chill surface. For convenience, this surface of the ribbon is hereafter called the "bottom" surface. Although out-of-contact quenching can be mitigated by casting under a vacuum (see e.g., U.S. Pat. No. 4,154,283, issued May 15, 1979, to R. Ray et al.), casting in air is more convenient.
After solidifying on the chill surface, resulting ribbon is either thermally or mechanically bonded to the surface. It must be stripped off to effect a continuous cast. Generally, the stripping process produces on the chill surface a plastically deformed track, whose roughness increases with progressive casting. Because of surface tension in the molten metal, as chill surface roughness increases, an increasing fraction of the ribbon is quenched out of contact with the chill surface.
When ferromagnetic metallic glasses are cast, the progressive increase in chill surface roughness may be accompanied by a deterioration in magnetic properties of the ribbon (after magnetic annealing). These properties include D.C. coercive force, H.sub.c ; remanence, B.sub.0 ; low-field (80 A/m) magnetization, B.sub.80 ; 60 Hz core loss and VA demand of toroidally wound cores. Beginning-of-run samples (produced on a relatively smooth chill surface) experience substantial improvement in all magnetic properties with magnetic annealing; end-of-run samples, produced on an eroded surface, show minimal improvement at best when annealed.
Removing a surface layer--for example, by chemical etching--to produce magnetic material of improved magnetic properties is disclosed in U.S. Pat. No. 1,998,840, issued Apr. 23, 1935, to V. E. Legg et al. That patent relates to crystalline materials that are first heat treated, and it generally involves removing from the surface more than 50 .mu.m ("2 mils").
Electrolytic etching of a magnetic thin film is disclosed in U.S. Pat. No. 3,575,825, issued Apr. 20, 1971, to R. E. Skoda. In that case, removing part of the film surface results in an increase in H.sub.c.